1. Field of the Invention
The present invention relates to a multi-walled steel pipe having a small diameter, typically less than 20 m/m, and providing sufficient resistance to pressure and mechanical strength, generally designed for use in an automotive vehicle or other machines and apparatus as an oil or gas feed pipe or a fuel injection pipe for a diesel engine.
2. Description of the Prior Art
As shown in FIG. 6, a conventional multi-walled steel pipe of this type includes a plurality of pipe bodies P1' and P2' of different diameters. Each body, a seamed or drawn pipe, has a flat peripheral surface 13 which is plated with copper to provide a brazing layer. The peripheral surfaces of the pipe bodies are overlapped as at 12 and brazed together as at 14.
However, in the prior art, the overlapping surface 12 is a flat surface. When the pipe bodies are heated in a furnace at a melt temperature of the brazing layer, part of the pipe bodies may not be brazed (namely, pinholes) due to the difference in wettability between the overlapping surfaces 12. This deteriorates its resistance to pressure and mechanical strength against cracking and breakage and thus, the quality of the pipe.